Hydrocyclones are used in the pulp and paper making industry for cleaning fibre pulp suspensions from contaminants, in particular but not exclusively from contaminants that differ from fibres in density. An important application is cleaning from contaminants in the form of heavy weight particles of a specific gravity greater than that of fibres, such as specks, shives, sand and metal particles in the size range of 100-1000 microns. The separation chamber of a conventional hydrocyclone designed for such an application normally has a diameter at the suspension inlet member smaller than about 150 mm to create centrifugal forces strong enough to pull the heavy contaminants radially outwardly in the vortex. The tapering design of the separation chamber is necessary to maintain the rotational speed of the vortex and, consequently, the required magnitude of the centrifugal forces acting on the heavy contaminants along the separation chamber, so that the separation efficiency is satisfactory throughout the separation chamber. In addition, maintaining the speed of the vortex is particularly important when cleaning high consistency fibre suspensions to prevent formation of fibre network. Such a fibre network negatively affects the separation efficiency and could plug the relatively small axial opening at the apex end of the separation chamber. Since the tendency of fibre network formation increases with increasing fibre concentration, the conventional hydrocyclone is normally used for separating fibre suspensions having a fibre concentration of up to 1.0%, in exceptional cases up to 1.5%.
A plurality of hydrocyclones of the conventional type coupled in parallel and forming a first separation stage has been employed in a conventional hydrocyclone plant to achieve the necessary total capacity for cleaning the large suspension flows, typically between 40 000 and 200 000 litres/minute, that often exist in the paper making industry. The conventional hydrocyclone plant also includes further separation stages of hydrocyclones of the conventional type, typically there are four to five stages coupled in cascade, to recover fibres from the reject fraction of the suspension developed in the first stage, whereby the separation efficiency of the plant is increased.
It is known to provide a hydrocyclone with a fluid injection member for injecting a flushing liquid into the separation chamber close to the vicinity of the reject fraction outlet to flush the thickened reject fraction so that fibres are released from the heavy contaminants and plugging of the reject outlet is prevented.